Will anyone ever run a sub-two-hour marathon?

Every morning, as dawn breaks across the shimmering blue mountains of Kenya's Great Rift Valley, a series of shadowy figures assemble at the various training camps dotted around the town of Iten. Gradually, some in clusters, some alone, they slip away for the first of the day's extended runs.

Ten years ago, these young athletes would have dreamed of winning gold medals on the track – but those days are long gone. The marathon has always been the holy grail for every distance runner, but it used to be the discipline they turned to once their track speed had faded. Nowadays, the financial rewards offered by the big city meets mean the marathon attracts the world's finest endurance specialists at the very peak of their powers. Perhaps one day, one of them will shatter one of sport's most imposing barriers: the sub-two-hour marathon.

The very idea of it appears almost inconceivable: 26.2 miles in 120 minutes, requiring an average speed of 13.1mph, something most of us would struggle to manage for 400m. "When I look at a two-hour marathon from a pacing and a physiological perspective, right now it seems really unlikely," says David Epstein, the bestselling author of The Sports Gene.

"A guy would need to have as high an oxygen capacity as has ever been recorded, and the best running economy [how efficiently you use oxygen while running at a given pace]. Those two things almost never seem to come together. And from a pacing standpoint, the elite guys usually run the first half of a marathon 2.5-3 minutes faster than the second half. So they'd have to run the first 13 miles significantly faster than has ever been done before."

Back in September, Kenya's Wilson Kipsang lowered the world record to 2:03:23 in Berlin. On the face of it, we would appear to be tantalisingly close – but then you have to consider that the record has only dropped by two minutes over the past 14 years.

Ethiopian distance king Haile Gebrselassie won two golds on the track before turning to the marathon at 29 and twice breaking the world record in his 30s. He believes that it may well take another 25 years before we see anyone capable of going under two hours. Uganda's Stephen Kiprotich, though, the current Olympic and world champion, believes there's room for optimism.

"Lowering the record to 2:02 will be done soon, definitely before the next Olympics in Rio," he said. "Then it will again take some time. The sub-two-hour marathon will happen, but it's impossible to set a timeline. But the good thing is that in the past few years, training camps in Africa have professionalised a lot. When I was a teenager we just trained; now I have a coach, a physio, a manager, a GPS watch. We have people during long runs who assist with drinks to simulate drinking station situations. All these things make a difference. But we still have lot of room to improve if you compare our training camps with some of the European and American runners. They have a lot of technology and knowledge we don't have."

Epstein believes that the feat is beyond the current generation, and suggests it may not even happen within our lifetime. Such are the physiological challenges that ultimately it will come down to the evolution of our genes.

"The faster the races are getting, the more people are ruled out purely because of their genetics," he said. "Oxygen capacity is what sets the pros aside from everyone else in the field, but what separates the pros from each other is usually running economy, and that's largely something that you're born with. You can improve what you have through working hard – and Paula Radcliffe was a great example of that. She improved her running economy with altitude training and she lost weight while she was still growing. However, there's still a certain body type that you naturally need to have, and of course you have to combine that with ferocious training."

But would it be possible to spot a runner with the ideal genetic makeup and nurture their potential from an early age? A lot depends on how the genes are expressed, which is one of the most fascinating and little understood areas in medical science.

"There's a lot of theory that living in a certain way turns the genes on and off, based on what the parents did when they were forming their germline DNA," Epstein explains. "This means what your mother was doing when she's pregnant with you and what your father was doing when he was going into puberty.

"But otherwise it's sort of unknown. I think there are certain lifestyle factors, specifically relating to runners in Kenya. A lot of them are primed for training, because they're not overweight and they grew up running, so they can start training really quickly instead of having to start by jogging. The first sub-two-hour marathon will probably be run by someone from east Africa."

We may have a while to wait, but whoever becomes the first to stop the clock at 1:59:59 will have accomplished one of the most significant achievements in sport. "It will be a great milestone and an indicator of what the human body can do," says Kiprotich's coach Patrick Sang. "It will show that what is impossible, can be possible."

Epstein believes that, in some ways, it may have a greater impact than the four-minute mile. "That was obviously huge, and part of the psychological rebuilding of post-war Britain," he said. "But the sport was also contained to a very small portion of the world at that point. Not that many people were competing in running, and not many countries recognised the mile in the same way that the United States and Britain do. But everyone recognises the marathon distance, and it's a globally competitive sport. The marathon has never been more popular – it's the greatest participator sport that we have going right now – so I think a lot of people could really relate to what it means."